Several unusual murine lymphoma lines were established and studied for antigenic and functional characteristics. The lines were derived from AKR/J strain mice and are noteworthy in that they exhibit characteristics of both T and B cells. When passaged into young normal AKR/J hosts the cells appear first like T-cell lymphomas and then differentiate into tumor cells with aspects of both T and B cells. Characteristics studied were Thy 1 antigen, Ia antigen, Fc receptor, complement receptor, mitogen responsiveness, and SIg. Factors controlling the expression of these markers in vivo and in vitro are being studied.